This invention relates to an automatic isocandle plotting system and, specifically, to an autoranging digital luxmeter for such a system. This application relates to application Ser. No. S87,603, filed concurrently herewith and assigned to the assignee of the present invention.
In the prior art, the testing of beam projector lamps, e.g., photographic projection lamps and automotive headlamps, has been a time-consuming and tedious process. Typically, the measurements are made manually by positioning the lamp, monitoring the voltage across the lamp, and recording the current from a photocell. The current from the photocell is then converted, by way of a look-up table, into illuminance or candlepower.
Depending upon the particular test being carried out, this procedure can take as long as 11/2 days for a single lamp. For example, the beam pattern of an automotive headlamp is given by what is known as an isocandle curve, in which a plurality of closed curves indicate the coordinates in space where the illumination is the same. To obtain these curves, the above procedure is repeated for each of a large number of points and the closed curves drawn to include the points.
Another test, involving fewer points but generally requiring 1 to 2 hours per lamp to complete, is the test for compliance with the S.A.E. (Society of Automotive Engineers) standards for headlamps. In this test, the beam pattern is checked at 14 locations for maximum or minimum illumination.
In general, it is desired to reduce the time required for these tests and to provide more precise information about the beam pattern. To this end, various systems have been proposed in the past. One such system utilizes a lamp positioning mechanism controlled by photocell detection circuitry. The circuitry monitors the current generated by a photocell and positions the lamp so that the photocell receives the same level of illumination as the beam is moved. Electric signals indicative of position control the position of a pen in a plotting mechanism to draw the isocandle curves. While faster than the manual procedure, the system is relatively slow. Further, the S.A.E. points are not evaluated except by chance.
Considering digital luxmeters per se, some commercially available units are capable of providing very accurate and precise readings of illumination, but at the expense of the time it takes to make the measurement.
In general, the luxmeters of the prior art do not provide a rapid measurement of a large number of points necessary to define a beam pattern, cannot respond to rapid changes in light levels, are incapable of interfacing with a computer and, in particular, are incapable of providing accurate, precise measurements at a rate sufficient to be used "on-line" with a computer, i.e., so the computer does not have to wait for information.